The equivalent circuit of a D.C. motor comprises a generator representing the back EMF, a resistance representing the armature and brush resistance, and an inductor representing the armature inductance. It is well known that a rotating motor acts like a generator due to the internal voltage it produces known as back EMF. However, when the motor is not rotating, the back EMF is zero and only the resistance and inductance remain.
In systems which use moving D.C. motors whose leads are connected to sliding collectors or contacts which ride in voltage supply rails, many additional challenges are presented to a failure detection technique for D.C motors which are not pertinent to stationary motors. It should be noted that in a system which uses moving D.C. motors, at any given point in time, each motor in the system is connected to a unique drive or supply system. The voltage supply rails are segmented with insulators inserted between sections of copper. Each copper segment of the rails is connected to individual stationary control means. In such a system, the motors are moved from segment to segment being energized when the application dictates it necessary.
In such a system, the resistance of each motor supply path is often larger than that of the motor. This fact makes it difficult to detect a short in either the motor, the motor leads, or the sliding collectors. However, the ferromagnetic structure of the motor makes its inductance the largest reactive element in the system, so that the presence of sufficient inductance offers a reliable method for verifying system integrity.
In order for a fault detection system to offer positive protection against potentially dangerous or damaging conditions, it should operate in real time and should function whether or not a given motor is being driven. In addition, when considering the aforementioned system, wherein D.C. motors are not stationary, the available methods of inductance sensing which can be employed are more restricted than they would be for a stationary or off-line measuring technique.
Since, as mentioned, the inductance of a motor is the largest reactive element in a motor circuit path, techniques which measure inductance can be employed in a fault detection system. A common method of measuring inductance is the use of an impedance bridge. It is very difficult to conduct bridge-type frequency domain inductance measurements for on-line, real-time applications.
The technique described herein, employs time-domain measurements which are easier to achieve and especially compatible with the commonly used pulse-width modulation (PWM) control systems for D.C. motors.
It should be noted that the system and method presented in this disclosure provide an accurate and reliable means for detecting failures in a D.C. motor circuit. It is not limited to, in any way, the aforementioned system where the motors are not stationary, this system and method can be applied to both stationary and non-stationary motors in many applications.